A fiber reinforced plastic (hereinafter also referred to as FRP) comprising reinforcing fibers and a matrix resin has gained greater attention in industrial fields because of having a high specific strength, a high specific modulus, good mechanical characteristics, and other highly functional characteristics such as weather resistance and chemical resistance, and demands for them is increasing every year.
The most widely used molding method for producing an FRP haying highly functional characteristics is an autoclave molding process in which a layered body produced by laminating a plurality of prepreg bases each of which is prepared by impregnating a sheet comprising continuous reinforcing fibers with a semi-cured matrix resin, is heated and pressed in an autoclave and the matrix resin is cured to mold an FRP.
The FRP produced by the autoclave molding process is composed of the reinforcing fibers being in continuous and therefore has good mechanical properties. Furthermore, where the continuous fibers are arranged regularly, such as in one direction, it will be easy to design an FRP having desired mechanical properties by arranging the prepreg base plates in an appropriate configuration or alignment, and the resulting FRP products will be small in variation of mechanical properties.
However, on the other hand, it is difficult to produce an FRP having a complicated shape such as three-dimensional, since the reinforcing fibers in the prepreg base are in continuous, and therefore, conventional FRP products have been limited to planar or nearly planar shapes.
A press molding process that uses a SMC (sheet molding compound) is one of the molding methods that serve to mold an FRP having a complicated shape such as a three-dimensional shape. In this molding process, chopped strands, normally cut into pieces of about 25 mm, are impregnated with a matrix resin to provide a SMC sheet comprising partially cured resin, which is then heated and pressed by using a press machine to produce an FRP. In most cases, the SMC is cut into sheets each of which is smaller than a molding FRP before the pressing, the sheets are placed on a mold, and the sheets are extended by pressing, or made to flow, into a desired shape. Thus, the flowing allows the material to be molded into a complicated shape such as a three-dimensional shape.
However, the chopped strands inevitably suffer uneven distribution and orientation during the SMC sheet forming step, and therefore, the resulting FRP product will have poor mechanical properties that vary largely over the product, causing some problems. Moreover, in the case of thin components, in particular, the molded products tend to suffer warp and shallow depressions etc. in the surface, often leading to decreased suitability as components of structural materials.
To eliminate such disadvantages in the above-mentioned FRP materials and their production processes, an improved FRP production method has been proposed in which deep cuts are put in a prepreg base that is composed of continuous fibers and a thermoplastic resin, in such a way that cuts are put in the continuous fibers in the direction across the continuous fibers, with the aim of increasing the flowability of the fibers during a molding process and decreasing the variation in the mechanical properties in the resulting moldings (JP 63-247012 A).
In the prepreg base described in JP 63-247012 A, however, cuts are simply put in the prepreg base though thermoplastic resin having a high melt viscosity is used as matrix, and therefore, if an attempt is made to produce a molded product having undulating portions, it will be impossible riot only to achieve such an undulating shape precisely but also maintain a high flowability of the prepreg base itself and that of the fibers in the prepreg base that are no longer continuous after being cut to limited lengths.
It could therefore be helpful to provide a method for producing a prepreg base having a good flowability of the prepreg base itself and/or fibers having a certain length and being not continuous which is prepared with cuts of fibers in the prepreg base, having a good followability to a shape of an intended mold, and having a wide range of selecting molding conditions, at the time of molding a mold.
It could also be helpful to provide a layered base that can be molded into an FRP product having excellent mechanical properties, quality stability, and appearance quality, and also provide FRP products that can be produced from the prepreg base or the layered base.